A large bearing for use in a large machine should last for many years, since it is an expensive part and is also difficult to access, often requiring the machine to be disassembled in order to be replaced. The main bearing of a wind turbine generator for example may be expected to have a useful lifetime of 20-25 years. For such large bearings, it is therefore very important to know how the geometry of the bearing might change during its lifetime. The main bearing of a large machine such as a wind turbine generator is often realised as a roller bearing, with essentially cylindrical rollers that transfer load from rotor to stator (or vice versa, depending on generator type) while allowing the rotor to rotate smoothly about the stator. To operate efficiently, it is important to ensure that the rollers are supported evenly along their lengths. The roller races (or “raceways”) must also be as even as possible so that the load is evenly distributed over the rollers.
A bearing specification for a large machine such as a wind turbine generator must ensure that these conditions are fulfilled over the expected bearing lifetime, and prescribes the use of material such as hardened or tempered steel and highly precise machining steps. Such a bearing can therefore be very expensive. However, even when manufactured to a very demanding specification, external influences that act on the bearing once it is operational may reduce its load-carrying capacity. For example, bolts may be used to fasten a bearing ring to another structure, and the act of tightening these bolts may distort the bearing slightly. Such a slight deformation of the bearing cannot be visually identified, and the bearing will continue to function. However, even only a slight deformation can change the support length of the rolling elements, and the reduced load-bearing capacity can significantly shorten the bearing lifetime.